Core B provides common facilities and expertise for mechanistic interpretation of pathological and immunopathological studies. The broad, long range objective of the Immunopathology Core B is to provide insights into the mechanism of action of the therapeutic manipulations used to induce grafts survival and tolerance. The insights from these studies will complement the interpretation of clinical samples in patients on similar protocols, as has been our experience. The other goal will identify and evaluate any toxicities and complications that may arise from these treatments. The primary role of the pathological analyses is to document and characterize the status of the graft (nature of infiltrate, presence of acute or chronic rejection, including a humoral component) and the systemic effects of the protocols (toxicity, complications). The mechanistic studies to be done on protocol biopsies include detection of Treg cells in grafts and lymphoid organs, measures of local inflammation in the graft and infiltration and activation of various cell types important in allograft rejection (T memory, dendritic cells) and antibody/complement effects. These are described in detail in the three projects, n addition full necropsies will be performed to determine the efficacy and adverse effects of the novel treatment strategies. The techniques that will be utilized include routine histology, immunohistochemistry, immunofluorescence with specialized techniques available as needed (electron microscopy, laser capture). Immunoperoxidase techniques with a panel of mAbs will distinguish the infiltrating cell types, adhesion and cytokine molecules and receptors, and activation markers quantitated using digital imaging of whole slides. Immunofluorescence will be used to localize the deposition of immunoglobulin and complement, as well as phenotyping cells with double/triple staining. C4d stains will be used to detect humoral rejection. We have a well-annotated tissue bank of frozen and paraffin embedded tissue available from our previous transplantation experiments (>6000 samples), which can be drawn upon to expand and refine our analysis.